High reverse breakdown voltage and low on-resistance are required for high current switching elements. Field effect transistors (FETs) that use a group III nitride semiconductor are superior in terms of high breakdown voltage and high-temperature operation because of their large band gap. In particular, vertical transistors that use a GaN-based semiconductor have been receiving attention as transistors for controlling high power. For example, there has been proposed a vertical GaN-based FET whose mobility is increased and whose on-resistance is decreased by forming an opening in a GaN-based semiconductor and forming a regrown layer including a channel of two-dimensional electron gas (2DEG) on the wall surface of the opening (PTL 1). In this vertical GaN-based FET, a structure including a p-type GaN-based semiconductor layer for improving pinch-off characteristics is proposed.